1. Field in the Industry
The present invention concerns an anode for oxygen evolution without forming chlorine in electrolysis of chloride-containing aqueous solutions including seawater.
2. Prior Art
In general, seawater electrolysis is performed to produce sodium hypochlorite by the reaction of chlorine formed on the anode with sodium hydroxide formed on the cathode in addition to the formation of hydrogen on the cathode. For this purpose, there has been used anodes made by coating titanium with an oxide or oxides of an element or elements of the platinum group (hereinafter referred to as “platinum group element(s)”) as the high performance electrodes.
On the other hand, like fresh water electrolysis to produce hydrogen and oxygen, for production of hydrogen and oxygen in seawater electrolysis, formation of hydrogen on the cathode and formation of oxygen on the anode without formation of chlorine are prerequisite, and hence, a special anode is required.
The inventors found the fact that the oxide electrode prepared by repeated coating of Mn salt solution together with Mo salt and/or W salt on a conducting substrate and subsequent calcination at high temperatures in air was active as an anode for oxygen evolution in electrolysis of sodium chloride solutions but inactive for chlorine evolution, and disclosed it (Japanese Patent Disclosure No. 09-256181). There are two types in this kind of electrodes:
(1) The electrode in which the conducting substrate is coated with the oxide containing 0.2-20 cationic % of Mo and/or W and the balance of Mn as the main cation.
(2) The electrode in which the conducting substrate is coated with the oxide containing 0.2-20 cationic % of Mo and/or W, 1-30 cationic % of Zn, and the balance of Mn as the main cation, and in which the effective surface area is increased by leaching out of Zn by immersion in a hot concentrated alkali solution.
The above-explained previous invention is based on the findings that, in production of oxygen evolution anode, calcination of Mn salt coated on the conducting substrate leads to formation of Mn2O3 and that inclusion of Mo and/or W in Mn2O3 enhances the oxygen evolution efficiency. In production of the oxygen evolution anode, if the calcination temperature is not sufficiently high, the stability of the electrode is insufficient due to insufficient crystal growth, but even at high temperatures Mn cannot be oxidized to the higher valence than three because of decomposition of high valent Mn compositions.
Nevertheless, the higher valence Mn oxide was expected to have the higher activity for oxygen evolution. Thus, Mn oxide formation was tried by anodic deposition from divalent Mn salt solution instead of calcination and a highly active anode consisting of tetravalent Mn was obtained. This finding was also disclosed (Japanese Patent Disclosure No. 10-287991). The method is novel in that anodic deposition is applied for production of this electrode consisting of the conducting substrate coated with the oxides containing 0.2-20 cationic % of Mo and/or W, and the balance of Mn as the main cation.
Subsequently, the inventors made the following inventions and the inventions were disclosed. They concern the electrolytic cell using the above-described anode (Japanese Patent Disclosure No. 11-256383), the electrode assembly using combination of the electrode and a diode (Japanese Patent Disclosure No. 11-256384), and a method of producing the anode (Japanese Patent Disclosure No. 11-256385). Furthermore, the inventors found that the electrode in which Fe is added to Mn—Mo, Mn—W or Mn—Mo—W oxides was effective as the oxygen evolution anode in the solutions containing chloride ion in a wide temperature range up to just below the boiling point of water (Japanese Patent Disclosure No. 2003-19267). Another patent application was filed for the modified technology of producing the anode including the preparation method of the titanium substrate (Japanese Patent Disclosure No. 2007-138254).
A common problem of oxygen evolution anodes is deterioration by prolonged operation. Insufficient adhesion of electrocatalyst to the substrate of the electrode leads to peeling off of the electrocatalyst due to high pressure of violently evolving oxygen gas. Furthermore, insufficient activity of electrocatalyst for oxygen evolution results in increase in electrode potential during oxygen evolution at a constant current density with a consequent acceleration of electrode deterioration due to formation of insulating titanium oxide. Thus, the stable anode on which oxygen evolution steadily occurs for a long period of time without deterioration of electrode during electrolysis of solutions containing chloride ion has been strongly demanded. Another requirement of oxygen evolution anode is high stability without degradation even if the anode is used in a wide pH range including a strong acidic condition.